Arc evaporator with a poweful magnetic guide for targets having a large surface area

ABSTRACT

Incorporating an anode ( 1 ) and a cathode or target ( 2 ), from which is obtained the evaporated material which will be applied to piece ( 10 ) to be coated within a vacuum chamber, and where with a magnetic guide it is endeavoured that the action of the electric arc on the target be displaced over the whole surface thereof, in a homogeneous way, the aforementioned magnetic guide is constituted by means of two independent magnetic systems, a first magnetic system constituted by a group of permanent magnets ( 3 ) located on the periphery of the cathode or target ( 2 ), in a disposition noticeably coplanar with the same, so that magnetization thereof is perpendicular to the surface of said target ( 2 ), and a second magnetic system constituted by an electromagnet ( 4 - 5 ), located in the rear part of target ( 2 ), housed in electrically insulating body ( 6 ) of the evaporator and at a certain distance from said target ( 2 ), with at least one of its magnetic poles parallel to the surface of said target ( 2 ), so that the combined action of the two magnetic systems determines uniform use or consumption of target ( 2 ), at the same time as an increased reliability of the evaporator.

OBJECT OF THE INVENTION

[0001] The present invention relates to an arc evaporator, that is amachine intended to evaporate a material, an electrical conductor, sothat said material, in vapour form, can be displaced within a vacuummedium in order to be deposited on the surface of the piece to be clad.

[0002] The object of the invention is to obtain an arc evaporator which,including a powerful magnetic guide, permits the cathodic point of thearc to be guided in an infinity of different trajectories, capable ofbeing individually selected and which encompass the whole surface areaof the target, for the purpose of achieving uniform use thereof. Also,the powerful magnetic guide causes a strangulation or narrowing of thecathodic point, which increases the temperature and ionisation of theemitted material, facilitating the obtaining of good quality coatings.

[0003] Furthermore, the powerful magnetic guide contributes to increasedreliability of the arc evaporator, by making it impossible for the arcto be displaced accidentally to a different point of the evaporationsurface area.

BACKGROUND OF THE INVENTION

[0004] As has already been remarked in the previous paragraph, arcevaporators are machines for evaporating a material, an electricalconductor, and emit it inside a vacuum bell jar in form of a vapourwhich can be displaced through the interior thereof. Usually thematerial to be evaporated is in plate form, one of the faces of which iscooled by water and the other is directed toward the interior of thevacuum bell jar, in a position facing the piece on which it is desiredto deposit the emitted vapours, the evaporation of the material beingcaused by provoking a direct current electric arc, of approximately 22volts and 80 amperes, between a cooled electrode which acts as anode andthe conductive plate which it is of interest to evaporate and which actsas cathode, a small quantity of gas necessary for maintaining the arcbeing habitually introduced additionally in the vacuum chamber.

[0005] In a more specific manner, the electric arc acts on the surfaceof the plate to be evaporated in a concentrated manner on a singlepoint, the cathodic point which is randomly displaced over the externalsurface area of the plate, which produces a not very homogeneousconsumption of the plate, or what is the same thing, good use is notmade of the constituent material of said plate, the cost of which isvery high.

[0006] To remedy this problem of lack of homogeneity in the consumptionof the plate it is endeavoured to control and direct the movement of theelectric arc, magnetic guides being used to such an end which producefields that can modify the track of the electric arc in a controlledmanner.

[0007] Different solutions exist at the present time for said magneticguides, all of them intended to control the movement of the arc on thecathode with the purpose of optimising homogeneous consumption, amongthose worthy of mention being the following ones:

[0008] U.S. Pat. No. 4,673,477 describes a magnetic guide which employsa permanent magnet which is displaced, by mechanical means, in the rearpart of the plate to be evaporated, in such a way that the variablemagnetic field which this permanent magnet generates produces a guidanceof the electric arc on the cathode. This machine also incorporatesoptionally a magnetic winding which surrounds the cathode plate for thepurpose of strengthening or weakening the force of the magnetic field ina direction perpendicular to the active surface area of the cathode soimproving the guidance of the electrode. The problem which this machinehas, is that the magnetic system of moveable permanent magnets is verycomplex mechanically and therefore susceptible to breakdown.

[0009] U.S. Pat. No. 4,724,058 relates to a magnetic guide whichincorporates some coils placed in the rear part of the cathode plate,which guide the electric arc in a single direction parallel to thatwhich the coil follows. For the purpose of reducing the effect ofpreferential consumption in a single track, methods are used whichendeavour to weaken the guidance effect of the magnetic field so thatupon the latter a random component is superimposed. In short, it hasbeen foreseen that the magnetic field generated by the coil is connectedand disconnected so that most of the time the arc is displaced on thecathode in a random manner, and a very small part is guided by themagnetic field. The problem with this machine is that, finally, theguidance takes place during a very short time and the rest is randomwhereby precise and efficient control of the consumption of the cathodeplate cannot be guaranteed.

[0010] U.S. Pat. No. 5,861,088 describes a magnetic guide which includesa permanent magnet located in the centre of the target and in the rearface thereof, and a coil which surrounds the aforementioned permanentmagnet, the assembly constituting a magnetic field concentrator. Thesystem is completed with a second coil placed on the exterior of theevaporator. The problem with this machine is that the magnetic fieldgenerated is weak and therefore also the guidance which it effects onthe electric arc.

[0011] U.S. Pat. No. 5,298,136 describes a magnetic guide for thicktargets in circular evaporators, which comprises two coils and amagnetic piece of special configuration which adapts to the edges of thetarget to be evaporated, in such a way that the whole works with asingle magnetic element, with two magnetic poles. Although thisconfiguration allows displacement of the track of the arc to a certaindegree, it is not capable of displacing the track thereof up to theexternal edge of the target or a small distance therefrom for which inorder to make efficient use of the material of the cathode, the guidehas to be sufficiently weak to allow a random component to besuperimposed on the movement forced magnetically.

[0012] In brief, all the systems of magnetic guides known present theproblem that if it is desired to obtain uniform consumption over thewhole surface area of the cathode, the arc has to be capable of movementwith a certain freedom and therefore weak guides should be used (ofreduced magnetic intensities), whereby it is not possible to maintainthe control over the track of the arc at all times.

[0013] If on the contrary very powerful magnetic guides are employed, itis not possible to achieve uniform consumption of the whole surface areaof the target or cathode.

DESCRIPTION OF THE INVENTION

[0014] The arc evaporator which the invention propounds resolves in afully satisfactory manner the problem outlined above, in each and everyone of the different aspects mentioned, through the incorporation of apowerful magnetic guide but which, by its special configuration, allowsthe cathodic point of the arc to be guided in an infinity of differentindividually selectable trajectories and which encompass the wholesurface area of the target, including the edges and centre thereof, auniform consumption of the target or cathode being achieved.

[0015] To this end and more specifically, said evaporator centres itscharacteristics in that its magnetic guide is formed by two independentmagnetic systems, that is, by four magnetic poles, which allows jugglingwith the magnetic intensity values of both systems and managingcancellation of the perpendicular component of the resultant magneticfield at the desired point of the surface area of the target, therebyachieving the guidance of the arc according to whatever track, from thecentre of the target to its outer edges.

[0016] Thus, by having a system which allows the guidance of the arc tobe guaranteed at all points of the target, it is possible to use strongmagnetic systems which therefore allow application of high magneticintensities on each of the trajectories, which causes an importantnarrowing of the cathodic point, which implies a major increase intemperature and degree of ionisation of the emitted material, greatlyfacilitating the obtaining of good quality coatings. It also increasesthe reliability of the evaporator, since the powerful magnetic fieldshold a point firmly to which the arc is applied on the selected track,avoiding the possibility of accidental displacement thereof over someunplanned area.

[0017] Specifically, the two magnetic systems which constitute the guidecan be a system of very powerful permanent magnets and a system ofelectromagnets, also very powerful, which guarantee a very high magneticintensity and therefore good control over the electric arc but, at thesame time it is possible to act on the electromagnets varying theirintensity, which produces a change in the track of the arc over thetarget. Therefore, with this system one obtains a strong magnetic guidewhich also allows variation, and therefore control, of the trajectoriesof the arc on the target achieving in this way uniform consumption ofthe whole surface area thereof.

[0018] The system of permanent magnets could also be substituted by asecond system of electromagnets, since the operation of the whole wouldbe similar.

DESCRIPTION OF THE DRAWINGS

[0019] To complete the description that is being made and with theobject of assisting in a better understanding of the characteristics ofthe invention, in accordance with a preferred example of practicalembodiment thereof, said description is accompanied with a set ofdrawings, as an integral part thereof, wherein are shown by way ofillustration and not restrictively, the following:

[0020]FIG. 1 shows a schematic representation in cross-section of arectangular arc evaporator with powerful magnetic guide embodied inaccordance with the object of the present invention.

[0021]FIG. 2 shows, also according to a schematic representation, a planview of the evaporator of the previous figure.

[0022]FIGS. 3, 4 and 5 reproduce the cross-section of FIG. 1, to whichin FIG. 3 a graph has been added of the vertical component of themagnetic field created by the externally located permanent magnets, inFIG. 4 a similar graph but corresponding to the magnetic fields createdby the electromagnet located in the rear part of the target whendifferent electric currents are applied in the electromagnet and in FIG.5 the magnetic fields created by the permanent magnets located on theexterior of the evaporator plus the electromagnet located in the rearpart of the target, likewise when different electric currents areapplied in the electromagnet.

[0023]FIG. 6 shows a similar cross-section to that of FIG. 1, accordingto a variant of embodiment in which the form of the electromagnetpermeable material has been altered.

[0024]FIG. 7 shows, finally, another cross-section similar to that ofFIG. 1 but corresponding to another variant of embodiment of theinvention wherein the permanent magnets on the exterior of theevaporator are each replaced by an electromagnet.

PREFERRED EMBODIMENT OF THE INVENTION

[0025] In the light of the figures mentioned, and especially of FIGS. 1and 2, it can be observed how in the arc evaporator which the inventionpropounds, there is an anode (1) and a cathode or target (2), as well asa magnetic guide, so that the direct current electric arc is formed,logically, between anode (1) and target (2) which acts as cathode ofsaid electric arc and therefore emits material from its surface area.

[0026] To assure that the point of target (2) on which the arc acts(point at which the perpendicular component of the magnetic field iszero) is displaced over the whole surface area of said target (2) in ahomogeneous manner, a magnetic guide is used comprising two independentmagnetic systems, a first magnetic system formed by a group of permanentmagnets (3) located on the periphery of the evaporator and so that theirmagnetization is perpendicular to the surface of the target, and asecond magnetic system constituted by a single electromagnet (4-5)located in the rear part of the target, at a certain distance from thelatter, the magnetic pole nearest to target (2) being parallel to thesurface of the aforementioned target.

[0027] The electromagnet (4-5) located in the rear part of target (2)consists of a core (4) of a material of high magnetic permeability andscant coercive force, such as wrought iron, surrounded by an electriccoil (5) which serves to generate the magnetic flux necessary tomagnetize the wrought iron, said core (4) having a rectangularcross-section, shown in FIG. 1, with the two magnetic poles arrangedparallel to the surface of target (2).

[0028] In more specific terms, electromagnet (4-5) is housed andperfectly fitted within body (6) of the evaporator, which has the formof a species of trough to the mouth of which is coupled cathode ortarget (2) which is secured with the collaboration of screws, not shown,electromagnet (4-5) being located below the target and at a certaindistance from the latter to guarantee that the magnetic field issufficiently homogeneous on the surface area of the target, a chamber(11) being defined in this way between target (2) and electromagnet(4-5) which can be used to emplace the systems necessary to assureappropriate cooling of target (2), as well as the rest of the componentsof the evaporator. As has been mentioned, the height of this coldchamber is determined by the need to have a certain distance between theupper face of the ferromagnetic core and the evaporation surface, sothat the magnetic field on the latter is sufficiently uniform.

[0029] The assembly is completed with an external base (9), some lateraland external hangers (8) and a barrier of slats (13) which forms a framewhich, as well as protecting the screws that fasten target (2) to body(6), confine the arc inside the target, so defining the evaporationsurface area. In turn, the insulating slats (13) are secured byappropriately electrically isolated screws (7′). All these elements,slats (13), hangers (8) and base (9) are obtained from materialselectrically insulating at high temperature, such as alumina,vitroceramics, boron nitride or PTFE, and which form an impediment forthe arc to form on undesired surfaces. These pieces require periodicmaintenance since in the course of arc operation they are graduallycoated with electrically conductive materials, whereby theireffectiveness for avoiding the formation of the arc diminishes.

[0030] Furthermore magnets (3) which configure the first magnetic systemon the periphery of body (6) of the evaporator, at the level of target(2), are implemented in external permanent magnets (3) which have to beof reduced height and located in such a way that the centre line thereofcoincides with the middle plane defined between the initial surface oftarget (2) and the surface which it will have at the end of its usefullife, also said magnets should be of the greatest possible power, forwhich reason they will be of maximum width and will be obtained on abasis of materials with high coercive force, like for example SmCo,NdFeB or hard ferrites.

[0031] The mechanism described is completed with a vacuum chamber orbell jar, not shown in the drawings, inside which is mounted piece (10)to be coated with the material evaporated from target (2).

[0032] In FIG. 3 a graph (12) is shown which corresponds to the verticalcomponent of the magnetic field created by permanent magnets (3) locatedon the exterior of body (6) of the evaporator and at a level with thesurface of target (2).

[0033] In FIG. 4, graphs (13) (14) and (15) shown, correspond to thevertical components of the magnetic fields created by electromagnet(4-5) located behind the surface of target (2) inside body (6) of theevaporator, when different electric currents are applied to saidelectromagnet (4-5).

[0034] Finally in FIG. 5 graphs (16) (17) and (18) are shown,corresponding to the vertical components of the magnetic fields createdon the surface of target (2) both by the permanent magnets located onthe exterior of the evaporator on the surface of target (2) and byelectromagnet (4-5) located in the rear part of target (2), whenapplying different electric currents to electromagnet (4-5).

[0035] As can be appreciated, this graph is the result of adding themagnetic field created by electromagnet (4-5) to the magnetic fieldcreated by permanent magnets (3) and gives as a result a verticaltranslation of the graph corresponding to the permanent magnets, so thatnow a part of this graph is situated in the positive part of thevertical axis. Essentially the cathodic point follows a track over thesurface of the target which is constituted by the points in which thevalue of the vertical component of the magnetic field is zero and thusfor example if the power applied to the electromagnet be adjusted insuch a way that the corresponding graph were that shown with number 16,the track of the cathodic point over the target would pass throughpoints 19 and 20, whilst if the current is adjusted so that thecorresponding graph was 18, the cathodic point would pass through points23 and 24 on the surface of the target in an intermediate track (17) thecathodic points would be 21 and 22.

[0036] As is observed in FIG. 6, in another practical embodiment, themagnetic core (4) can have a T-shaped section with one of its polesparallel to target (2) and the other perpendicular to the target, thisconfiguration allowing a greater intensity to be obtained of themagnetic field on the surface of the evaporator, as well as a greaterextension of the magnetic field in the horizontal plane, which allowsthe distance to be reduced between the upper face of ferromagnetic core(4) and target (2).

[0037] Lastly, it is pointed out that permanent magnets (3) could bereplaced by some electromagnets (3′), of similar structure to that ofelectromagnets (4-5), as may be observed in the embodiment shown in FIG.7.

1. Arc evaporator with a powerful magnetic guide for targets having alarge surface area, which being of the type of those which incorporatean anode (1) and a cathode or target (2), housed within a vacuum belljar, in which is also housed a piece (10) to be coated with materialevaporated from target (2) by means of an electric arc produced betweenanode (1) and cathode (2), in which also participates a magnetic guideto control and direct the movement of the electric arc, is characterisedin that said magnetic guide is constituted by two independent magneticsystems, that is, four magnetic poles which allow the resultant magneticintensity to be varied at will so that the electric arc follows thetrack desired on the target, one of the magnetic systems beingconstituted by a group of permanent magnets (3), located on theperiphery of the evaporator, in a disposition coplanar with target (2),with its magnetization perpendicular to the surface of said target (2),and the second magnetic system constituted by an electromagnet (4-5),located in the rear part of the target (2), at a distance from thelatter, the upper magnetic pole or that nearer target (2) being arrangedparallel to the surface of the aforementioned target (2).
 2. Arcevaporator with a powerful magnetic guide for targets having a largesurface area, according to claim 1, characterised in that electromagnet(4-5) corresponding to the second magnetic system is housed within body(6) of the evaporator, which constitutes a species of trough to themouth of which is secured target (2), which is sufficiently distancedfrom electromagnet (4-5) to permit the installation between theseelements of the cooling systems of target (2), the target being securedto body (6) with the collaboration of the relevant screws and on these,constituting kind of perimeter frame, a barrier of slats (7) of a hightemperature insulating material, body (6) being finished with aplurality of hangers (8) and a rear base (9), all of which pieces beingof materials electrically insulating at high temperature such asalumina, vitroceramic, boron nitride or PTFE.
 3. Arc evaporator with apowerful magnetic guide for targets having a large surface area,according to claim 1, characterised in that electromagnet (4-5)constituting the second magnetic system, is structured on the basis of acore (4) of high magnetic permeability and scant coercive force, such aswrought iron, located in correspondence with the central area of theevaporation surface, an electric coil (5) being arranged around it whichserves for generating the necessary magnetic flux to magnetize core (4).4. Arc evaporator with a powerful magnetic guide for targets having alarge surface area, according to previous claims, characterised in thatthe aforementioned magnetic core (4) has a rectangular cross-sectionwith the two magnetic poles arranged parallel to the surface of target(2).
 5. Arc evaporator with a powerful magnetic guide for targets havinga large surface area, according to claims 1, 2 and 3, characterised inthat core (4) of the electromagnet has a T-shaped cross-section, in sucha manner that one of its poles is parallel to target (2), the other polebeing perpendicular to the aforementioned target (2).
 6. Arc evaporatorwith a powerful magnetic guide for targets having a large surface area,according to claim 1, characterised in that permanent magnets (3) are ofreduced height and are located so that the centre line of the magnetscoincides with the middle plane defined between the initial surface oftarget (2) and the surface which it will have at the end of its usefullife, presenting the greatest possible width to achieve maximum powerand they will be obtained from materials of high coercive force, likefor example SmCo, NdFeB or hard ferrites.
 7. Arc evaporator with apowerful magnetic guide for targets having a large surface area,according to claim 1, characterised in that the two magnetic systems areconstituted by electromagnets.